Flat panel display devices are widely used in today's mobile, computer centric society. These displays are typically formed on rigid substrates and use a variety of light modulation techniques (such as liquid crystal display, organic light emitting diodes, and plasma) to form images. These displays suffer from limitations in substrate size due to manufacturing technology, materials, and interconnection limitations, and limitations in application due to the use of rigid substrates.
Display devices having flexible substrates are under development by a wide variety of companies. However, these displays are typically limited in performance due to the difficulty of forming active matrix switching circuitry on flexible substrates. Such flexible switching circuits are not yet commercially available; see for example U.S. Pat. No. 6,551,717 B2 issued Apr. 22, 2003 to Bell et al., describing an organic semi-electrode film fabricated by applying a solution containing an organic semi-electrode material and a solvent to a substrate. The resultant organic semi-electrode film contains a large area that exhibits a relatively high charge carrier.
Alternatively, passive matrix display designs that do not include switching circuitry on the display substrate may be used but are limited to a smaller number of light emitting elements and are not useful for video capable, full color displays. The brightness of passive matrix displays is also limited, particularly for large displays, by limited electrode conductivity and the need for high current and refresh rates to reduce display flicker. For example, the resolution of passive matrix OLED displays is typically limited to about 100-200 rows for 100 candelas/m2 display brightness levels.
In a passive matrix display, only one row at a time is active, the rows are sequentially activated at a high frequency to create the illusion of a continuous display. As a consequence, the pixels must be very bright when activated to maintain the brightness of the display and to prevent the appearance of display flicker. In order to achieve this brightness, the row and column electrodes must have high conductivity to minimize voltage drops along the row electrode. Either the row or column electrodes must be transparent above or below the pixel to allow light to escape from the pixel. This transparency limits the current carrying capability of the electrode. The role of the row and column electrodes can be interchanged.
US 2002/0196402 A1 by Sanford et al., published Dec. 26, 2002, describes an OLED display that includes a substrate, a display element disposed on the substrate, the display element having: a first electrical electrode; a second electrical electrode; a light switching material disposed between the first electrical electrode and the second electrical electrode; and a via through the substrate for electrically coupling a signal to the first electrical electrode. However, this approach to solving the difficulties of an OLED passive-matrix display is not suitable for a flexible substrate since the use of vias imposes manufacturing problems such as difficulty in hermetically sealing the display.
The use of conventional electronic components on a flexible substrate to form flexible computers is known. For example, US 2003/0048256 A1 by Salmon, published Mar. 13, 2003, describes techniques for building lightweight computing devices that may weigh less than one pound. The computing device may include a motherboard, a keyboard, and a display. Alternatively it may include a motherboard, a display, and speech processing capabilities. The motherboard is preferably built on a flexible substrate using a rigid carrier. IC (integrated circuit) chips are attached using flip chip bonds that employ stud bumps on the IC chips, and corresponding wells filled with solder on the motherboard. However, such a system uses a motherboard with additional boards, increasing the size of the system and the expense of manufacturing, and does not address problems with flexible displays.
The use of organic light emitting diode technology on a flexible substrate to form a display is also known. For example, US 2003/0034497 A1 by Yamazaki et al., published Feb. 20, 2003, describes a light emitting device including an OLED formed on a plastic substrate, where the plastic substrate comprises a plurality of barrier films and a laminate structure. This technology does not address the problem of display interconnection or switching circuitry on a flexible substrate.
It is also known to combine a flexible substrate with conductive elements connecting small, integrated circuits. For example, US 2002/0061392 A1 by Jacobsen et al., published May 23, 2002, describes apparatuses and methods for forming displays. One embodiment of the invention relates to depositing a plurality of blocks onto a substrate and coupling a flexible layer having interconnect deposited thereon. Another embodiment of the invention relates to forming a display along a length of a flexible layer wherein a slurry containing a plurality of elements with circuit elements thereon washes over the flexible layer and slides into recessed regions or holes found in the flexible layer. Interconnect is then deposited thereon. In another embodiment, interconnect is placed on the flexible layer followed by a slurry containing a plurality of elements. However, this design requires a multiplicity of light emitting circuit elements that reduces the resolution and increases the size of a display and requires a more complex manufacturing process.
There is a need therefore for an improved flat panel flexible display device.